1. Field of the Invention
This invention generally relates to a sensing method and apparatus and, more specifically, to a physical value sensor for use in an explosive atmosphere.
2. Background Description
A sensor is made of resistive or capacitive impedances related to a physical value, i.e., pressure, temperature, flow, etc., to be measured in an environment. This kind of sensor is an electrically passive element, that is to say, the sensor needs to be driven by an external power supply. A sensor signal processor receives a signal response from the sensor and generates a measured output. In the case where the sensor must be put in a hazardous area, the sensor signal processor is designed to provide protection against high currents or voltages to the sensor.
A hazardous area is defined as an area in which explosive atmospheres are, or may be expected to be, present in quantities such as to require special precaution for the construction and use of electrical equipment. Intrinsic safety is a protection concept employed in applications involving a potentially explosive atmosphere. Intrinsic safety relies on the electrical apparatus being designed so that it is unable to release sufficient energy, by electrical means, to cause an ignition of a flammable gas, for example. In electrical circuits, the mechanisms for the release of this ignition energy are an open circuit or a short circuit of components, interconnections in a circuit, and so on.
To realize intrinsic safety, a safety barrier may be designed in a non-hazardous area by relatively large circuits with energy limiting resistors, zener diodes, fuses, and so on. Then there were some problems in distinguishing the true impedance of the sensor from current limiting impedances, and imprecise measurement is caused by the noise effects chiefly coming from temperature characteristics of those impedances.
A technology to realize intrinsic safety by blocking capacitors has been tried in another technical field, specifically that of a microwave band antenna barrier invention as described in Japanese laid open application JP10-013130. A blocking capacitor is known to be effective to take care of an alternating current (AC) modulated signal since it blocks direct current (DC) voltages. However, in this application, the sensor has been ordinarily driven by DC power, which treats a small change of physical value, and it has been difficult to avoid its large conductance change noise.
Heretofore, it has been easy and certain to install a safety zener barrier so as to cover a sensor device as a whole from the external layer a power supply belongs to. But such an instrumentation that requires an external barrier may be a big thing because of wiring cost and space factors for mounting.
In view of foregoing, it is an object of the present invention to provide a sensor signal processing circuit which itself contains a safety barrier that limits the current energy so as not to ignite an explosive environment where a sensor element exists, especially by making the safety barrier low cost and compact enough to be mounted in a non-hazardous area or in an enclosure in the hazardous area. This safety barrier allows for the sensor signal processing circuit that would otherwise not be able to achieve intrinsic safe level approvals to attain such approvals standardized by official agencies.